用于模式搜索的 Rabin-Karp 算法的Java程序

给定一个文本txt[0..n-1]和一个模式pat[0..m-1] ，编写一个函数search(char pat[], char txt[])打印txt中所有出现的pat[] [] 。你可以假设 n > m。

例子：

输入：txt[] = "THIS IS A TEST TEXT" pat[] = "TEST" 输出：在索引 10 找到的模式 输入：txt[] = "AABAACAADAABAABA" pat[] = "AABA" 输出：在索引 0 找到的模式在索引 9 找到的模式 在索引 12 找到的模式

朴素字符串匹配算法会一张一张地滑动模式。每张幻灯片后，它会逐一检查当前班次的字符，如果所有字符都匹配，则打印匹配项。
与朴素算法一样，Rabin-Karp 算法也将图案一一滑动。但与 Naive 算法不同的是，Rabin Karp 算法将模式的哈希值与当前文本子串的哈希值匹配，如果哈希值匹配，则只有它开始匹配单个字符。所以 Rabin Karp 算法需要为后面的字符串计算哈希值。

1) 图案本身。
2) 长度为 m 的文本的所有子串。

Java

// Following program is a Java implementation
// of Rabin Karp Algorithm given in the CLRS book
  
public class Main {
    // d is the number of characters in the input alphabet
    public final static int d = 256;
  
    /* pat -> pattern
        txt -> text
        q -> A prime number
    */
    static void search(String pat, String txt, int q)
    {
        int M = pat.length();
        int N = txt.length();
        int i, j;
        int p = 0; // hash value for pattern
        int t = 0; // hash value for txt
        int h = 1;
  
        // The value of h would be "pow(d, M-1)%q"
        for (i = 0; i < M - 1; i++)
            h = (h * d) % q;
  
        // Calculate the hash value of pattern and first
        // window of text
        for (i = 0; i < M; i++) {
            p = (d * p + pat.charAt(i)) % q;
            t = (d * t + txt.charAt(i)) % q;
        }
  
        // Slide the pattern over text one by one
        for (i = 0; i <= N - M; i++) {
  
            // Check the hash values of current window of text
            // and pattern. If the hash values match then only
            // check for characters on by one
            if (p == t) {
                /* Check for characters one by one */
                for (j = 0; j < M; j++) {
                    if (txt.charAt(i + j) != pat.charAt(j))
                        break;
                }
  
                // if p == t and pat[0...M-1] = txt[i, i+1, ...i+M-1]
                if (j == M)
                    System.out.println("Pattern found at index " + i);
            }
  
            // Calculate hash value for next window of text: Remove
            // leading digit, add trailing digit
            if (i < N - M) {
                t = (d * (t - txt.charAt(i) * h) + txt.charAt(i + M)) % q;
  
                // We might get negative value of t, converting it
                // to positive
                if (t < 0)
                    t = (t + q);
            }
        }
    }
  
    /* Driver program to test above function */
    public static void main(String[] args)
    {
        String txt = "GEEKS FOR GEEKS";
        String pat = "GEEK";
        int q = 101; // A prime number
        search(pat, txt, q);
    }
}
  
// This code is contributed by nuclode

输出：

Pattern found at index 0
Pattern found at index 10

有关更多详细信息，请参阅有关模式搜索的 Rabin-Karp 算法的完整文章！

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